In an effort to provide vehicles with an increase in braking force, a power brake booster is disclosed in U.S. Pat. No. 3,603,209 in which the hydraulic fluid utilized in the operation of a power steering apparatus may be selectively diverted to operate a power braking device. In this device a lever mechanism carried on a boost piston will respond to an input force to proportionally reduce the output of a hydraulic pump from the power steering apparatus to the power brake apparatus. In such a power boost, it may be possible to deplete or reduce the hydraulic fluid being communicated to the power steering apparatus sufficiently to cause difficulty in maneuvering a vehicle on a curving road.
Similarly in U.S. Pat. No. 3,639,008 the brake pedal is pivotally located on the power boost device to proportionally activate a valve to allow stored hydraulic fluid to be communicated to the wheel brakes to stop a vehicle. However, in such hydraulic devices the pump for pressurizing the hydraulic fluid must be sized to adequately operate both the power braking apparatus and the power steering apparatus. Therefore, there has been reluctance on the part of car manufacturers and the buying public to change either from manually operated or pneumatic pressure differential power assist brakes. However, recent congressional legislation sets forth specific stopping requirements which have limited the vehicles in which manual or pneumatic operated brakes will be acceptable.